Mensuration devices for industrial applications utilize a variety of different techniques for determining the relative position of two objects with respect to one another. Such devices are especially useful in the machine tool and robotics industries for determining the relative position of two moving parts. As an example, it is highly desirable to accurately know the position of a cutting tool with respect to a lathe bed, or a milling head with respect to a workpiece for high-precision machining. Electronic and optical encoding systems are available which can interface with the electronic control system of automated machine tools so that such tools can perform pre-programmed operations on a workpiece with relative position feedback. Much higher machining accuracies can be achieved with a technique of this type, rather than merely relying on pre-programmed positioning of a cutting head in an open-loop system. In the robotics industry, it is clearly desirable for industrial robots to know the relative position of arms, manipulators, etc., with respect to the robot body.
There is an increasing need in the industry for Absolute measurements in encoders used for measurement and control. (Hereafter, absolute measurement, meaning that a reference scale includes a unique information content at each possible measuring coordinate, will be referred to as "ABS" measurement). A common solution for this is optical encoders with several binary related tracks. For instance, for a measurement range of 1 meter and a resolution of 1 micron, about 17 encoder tracks are needed for an Absolute scale of this type, (Finest track with 20 micron wavelength, interpolated down to 1 micron plus 16 tracks for binary definition of coarse ABS position on the scale). The detector for those sever tracks requires 4 photoelements each for accurately detecting the position within each track. This means 17*4=68 photoelements with associated electronic circuits, whose relative sensitivity and DC offset have to be adjusted for uniformity within each channel. There are cost and reliability penalties with such an optical ABS system.
There also exists a capacitive displacement sensor system (U.S. Pat. Nos. 4,879,508 and 4,878,013) which is capable of measuring ABS position over a large relative range. That system has simple detector elements (capacitive electrodes on a printed circuit readhead) and does not require any critical adjustment for reaching a high interpolation rate, and can cover a large relative range with only three channels. Its limitation is the ability to work with small enough scale pattern wavelength, and high enough interpolation rate, to reach resolutions of 1 micron or lower. A small pattern wavelength will require a small well controlled gap between scale and detector head, which can be impractical and costly for long scales.
A single channel optical system has, on the other hand, a potential of reaching improved resolution down to fractions of microns by utilizing shorter wavelength on the scale pattern and higher interpolation rate without severe cost penalties. There also are several possibilities to design the optical system with diffraction-based detector heads, that can resolve a fine scale pitch with a relatively large gap between scale and detector head.
Such a single channel optical system can work well with a scale pattern of wavelength on the order of 20 microns. Interpolating by a factor of 20 then gives 1 micron resolution. Interpolating by a factor of 100 gives a resolution of 0.2 micron. The problem is, however, that a perfectly infallible counter is required in such a system, for measurement of displacements greater than 20 microns. Otherwise, there is an ambiguity in the measured value, of (N * 20 microns). Such an ambiguity will occur following any disruption of the "infallible counter", such as that due to a power failure, or electrical interference, etc.
Such a result would have disastrous consequences in a fully-automatic machine tool unless the machine tool were programmed to shut down in the event of a power failure, transient power spike, etc.
Typical optical systems are described in U.S. Pat. No. 4,218,615, to Zinn, Jr., and U.S. Pat. No. 3,812,352, to MacGovern. These systems rely on the generation of point sources from a conventional light source through use of grating, etc. These point sources are reflected off or passed through an optical pattern on a scale portion. The scale pattern itself may form a second grating so as to establish a periodic pattern using the point sources generated by the light source first grating. Very high resolutions can be achieved with systems of this type. However, these optical systems do not know the relative displacement of the scale and pick-off portions if power is interrupted, or if the relative portions move too quickly for the electronic counters.
Thus, a need exists for a more economical, reliable, and compact absolute position encoding system which has high resolution and which is not limited by reliance upon counter the number of steps which a pick-off or transducer has been displaced from an origin on a scale.